Milling one or more strings in an uphole direction in a single trip are discussed in U.S. Pat. No. 8,555,955. Uphole milling is also described in U.S. Pat. No. 6,679,328. Grinding cuttings moving uphole from a mill for a second time to reduce their size so that they can be circulated out of a borehole are described in US 20160040496 and 20160040495. Drilling systems that monitor parameters such as fluid circulation rates as well as borehole parameters such as rat hole depth as well as a cuttings removal rate to allow real time changing of drilling parameters are described in US 20140209383.
When milling in an uphole direction the cuttings are allowed to go to hole bottom frequently referred to as the rat hole. If the cuttings fall to hole bottom as planned there is no problem later with plugging and abandoning the borehole with cement placed within the section of hole that was removed by milling. On the other hand, if the cuttings bridge the borehole close to the mill location, the milling itself can be affected or the position of the bridge can impact the ability to place cement so that the well will be not properly sealed when the cement is pumped into position where the casing was milled out.
The present invention seeks to address this issue in several ways. The uphole mill assembly has a signaling capability to determine whether or not a bridge is forming and if the bridge is forming the system can detect its location and its density in real time. The uphole milling assembly contains a downhole oriented mill or similar device that can be brought against the bridge to grind up the bridge so that uphole milling can resume. These and other aspects of the present invention will be more readily apparent from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.